1. Field of the Invention
The present invention relates to a driving circuit of a motor.
2. Description of Related Art
A system of speed control feedback has been generally used as a driving circuit for driving a brushless motor at high speeds. This system of speed control feedback is a kind of feedback system which compares a speed command value with an actual detected value using a control amplifier and which controls a motor (brushless motor) to be controlled so as to eliminate an error between them.
In this respect, the speed of the brushless motor can be detected based on the output of a Hall effect sensor for detecting the polarity of the field magnet of a rotor mounted on the brushless motor.
In general, the brushless motor is widely used when a constant speed is required and, when high speeds and low speeds are required, a brush motor is used because it is easily driven at variable speeds. In other words, a problem is produced that, when the above-described brushless motor is controlled by using a speed control feedback system based on the output of the Hall effect sensor and the brushless motor is driven at low speeds, because the output of Hall effect sensor has a small amount of information, a response to the feedback is reduced to make the rotation unstable and to reduce the rotation because of a variation in load. In order to solve this problem, the brushless motor can be provided with an encoder for detecting the position of rotation, in addition to the Hall effect sensor for detecting the polarity of the field magnet, to increase the amount of information relating to the speed control, but the cost of the brushless motor is greatly increased if the brushless motor is provided with an expensive encoder only for speed control.
The driving circuit of a three-phase brushless motor relating to a related art will be described with reference to FIG. 12.
The above-described driving circuit is constituted by providing six driving elements comprising field effect transistors FET41 to FET46 with switches SW1 to SW6. In this respect, when a current is passed to a U-phase--a V-phase, the FET41 is turned on by the SW1 and the FET44 is turned by the switch SW4. At the same time, when a current is passed to a V-phase--a W-phase, the FET43 is turned on by the SW3 and the FET46 is turned by the SW6. Further, when a current is passed to a W-phase--a U-phase, the FET45 is turned on by the SW5 and the FET42 is turned by the SW2.
In this respect, as shown in FIG. 12, the switches SW1 to SW6 (comprising transistors or the like, for example) for turning-on/off the FET41 to FET46 needs power circuits V1 to V6, respectively. In this respect, since the switches SW2, SW4 and SW6 for turning-on/off the lower side FET42, FET44 and FET46 are connected to the ground, the power sources V2, V4 and V6 can be shared but four power circuits are required.
In addition, six semiconductor switches are required for exciting three phases of U, V and W and the switches SW1 to SW6 (comprising transistors or the like, for example) for driving these six semiconductor switches need to be provided with a control signal, respectively: that is, a circuit for producing the control signals for three phases of U, V and W.
The present invention is made to solve the above-described problems. It is an object of the present invention to provide a driving circuit which can drive the brushless motor in a wide range of rotation.
It is another object of the present invention to provide a motor driving circuit for controlling an FET by a single power source.
It is further another object of the present invention to provide a motor driving circuit which can restart a motor quickly after an electric power supply is stopped for a long time.
It is still further another object of the present invention to provide a driving circuit for a three-phase brushless motor which can drive a motor in a wide range of rotation with a simple constitution.